This invention relates to a picture signal coding apparatus which is of particular utility when employed in the coding of still pictures.
For the reduction of the cost of picture signal transmission, there have heretofore been proposed DPCM, .DELTA.M and like coding systems, for example, in A. Habihi and G. S. Robinson "A Survey of Digital Picture Coding" Computer, 7,5, pp. 22-34 (May 1974). These known systems are called prediction coding systems, in which a prediction signal is produced based on a sample preceding a reference point and a prediction error signal which is a difference between a sample to be coded and the prediction signal is quantized for transmission. Due to the property of the picture signal, the prediction error signal occurs frequently in the range in which its amplitude is small, so that even if the prediction error signal is quantized roughly, the picture quality is not so much deteriorated; therefore the number of bits necessary for coding can be reduced as compared with that in ordinary PCM in which a sample is coded as it is.
With these conventional prediction coding systems, however, the prediction error signal must be transmitted every sample point, and the number of prediction error signal quantizing levels cannot be reduced extremely because of contributing factors to deterioration of the picture quality such as slope over load, granular noise, false contour, etc. For these reasons, the number of bits used is relatively large as a whole.
As a result of novelty search conducted by the European Patent Office, there was not found any particular prior literature corresponding to this invention but U.S. Pat. Nos. 3,403,226, 3,940,555 and 3,984,626 were cited as reference literatures. Now, a description will be given of U.S. Pat. No. 3,403,226 of the type in which a picture frame is divided into blocks. In this patent, one of picture elements is selected as a typical picture element for each block and coded by the PCM technique so that it can represent, by itself, its level, and the difference between the typical picture element and each of the other picture elements of the same block is coded into a PCM code.
Accordingly, though the number of quantizing levels for the latter PCM code is smaller than that for the typical picture element, transmission of the PCM codes requires a plurality of bits, resulting in an appreciably large number of bits being needed as a whole.
Some of the present inventors have proposed a picture signal coding apparatus of the type in which a frame of a gray-scaled picture is divided into a plurality of blocks; for each of the blocks, a threshold value is determined based on the luminance distribution in the block; each picture element signal of the block is compared with the threshold value to obtain a resolution component 1 or 0 according to the magnitude of the picture element; based on the resolution component and the picture element signals of the block, two typical luminance levels, that is, gray components, are determined; and the gray components and the resolution component are provided as coded outputs for the block. With such a method, two typical luminance levels are provided for each block and a 1-bit resolution component for each picture element; therefore the number of bits used in small as a whole and the picture can be reconstructed with excellent minuteness and tone wedge property. In this case, however, coding errors differ with blocks due to the property of the picture whose changes differ throughout its entire area of the frame. In other words, the coding errors are not distributed uniformly all over the reconstructed picture. Further, there is the possibility that the coding errors are centered on the areas near the borders of the blocks to make contours of the blocks noticeable in the reconstructed picture.
An object of this invention is to provide a picture signal coding apparatus which enables coding with less bits than those used in the prior art.
Another object of this invention is to provide a picture signal coding apparatus in which a picture frame is divided into blocks and then coded but the contour of each block is not noticeable in the reconstructed picture.
Still another object of this invention is to provide a picture signal coding apparatus in which a picture frame is divided into blocks but their sizes are each changed in accordance with the local property of the frame, thereby to enable coding with less bits.